Why is vertex drawing ignoring grid snap settings?
If I want to draw some vertices on the ground plane, I seem to have to
  place the first vertex with Ctrl-RMB (not Ctrl-LMB as the manual
  states...


That's probably because Blender used RMB for selection by default prior to 2.8. WWas it on the 2.8 manual or the 2.79?

For your issue you have to extrude, cancel the moving of the extrude by using right clic mouse button, then move it with G (dont deselect anything by error) to get the snapping effect.